loadUrl enhancements (#134)

`loadUrl` enhancements

- Make `options` parameter optional
- Allow partial overriding of instance options when calling `loadUrl` directly
- Make `requestOptions` optional
- Document `loadUrl` usage and provide examples

lib/util/extend.js

@@ -0,0 +1,21 @@
+module.exports = function(target) {
+  if (target == null) {
+    return target
+  }
+
+  var to = Object(target)
+
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i]
+
+    if (source != null) {
+      for (var key in source) {
+        // Avoid bugs when hasOwnProperty is shadowed
+        if (Object.prototype.hasOwnProperty.call(source, key)) {
+          to[key] = source[key]
+        }
+      }
+    }
+  }
+  return to
+}